[css-mixins-1] Remove superfluous </pre> tags

Author: andruud

css-mixins-1/Overview.bs

@@ -446,7 +446,6 @@ If this happens, then declarations appearing later win over earlier ones.
 	</pre>
 	The value of the '@function/result' descriptor of <code>--mypi</code>
 	is <code>3.14</code>.
-	</pre>
 </div>
 
 <div class='example'>
@@ -458,7 +457,6 @@ If this happens, then declarations appearing later win over earlier ones.
 	</pre>
 	[=Local variable=] descriptors may appear before or after
 	they are referenced.
-	</pre>
 </div>
 
 Conditional Rules {#conditional-rules}